A highway transmits vehicular traffic as plural discrete (but often almost contiguous) advancing highway traffic capsules each of which comprises plural vehicles which remain relatively static within the respective advancing traffic capsules as the latter are transmitted along the highway. Efficient advancement of a highway traffic capsule, in the sense of maximum safe vehicle volume passage per unit time, requires a balance between vehicle count in the capsule and the speed with which the capsule advances. Efficient traffic flow along a highway requires this balance to be achieved for all traffic capsules on the highway.
In almost all countries, vehicles are largely driven by discretionary driving with enforcement against unacceptable forms of discretionary driving applied punitively as a deterrent which discourages, with varying degrees of effectiveness, only those practices which are considered a threat to highway safety. Advancement of highway capsules in real conditions differs greatly from that model advancement because of the wide freedom of choice which can be exercised by the drivers of different vehicles in practising discretionary driving (regardless of whether choices are exercised on the basis of considered responses to perceived or real driving conditions or on the basis of random discretionary choice dependent upon mood, personal requirements and driver inter-relationship). Vehicle count in a highway capsule, for example, depends on such factors as driver perceptions of safe inter-vehicle distance, visibility, traffic volume pressure and individual speed requirements for the journeys in which individual vehicles and their drivers are engaged. The speed of advancement of a highway capsule is dependent on similar factors, and the two are obviously inter-dependent.
Discretionary driving leads to the presence on a highway of a complex array of different vehicle travel characteristics each resulting from a combination of individual driver attitudes and the influence on them of other driver attitudes and real conditions. That array produces the divergence between real traffic flow and model flow which is experienced in practise.
The array of different vehicle travel characteristics is also responsible indeed for the capsular configuration of highway traffic. A lead vehicle on a highway travels at a speed and in an overall manner decided by the driver of that lead vehicle exercising freedom of choice on the basis of considered responses to perceived or real driving conditions and/or on the basis of random non-rational discretionary choice. That vehicle obliges common lane following vehicles to travel in a manner influenced by the lead vehicle, in particular with respect to speed of travel, with the result that the following vehicles form with the lead vehicle a traffic capsule which advances along the highway but in which the individual vehicles are essentially in stasis with respect to the capsule. Ahead of the capsule are plural other vehicles which form a separate traffic capsule whose travel characteristics are determined by its own lead vehicle. The latter capsule should theoretically merge with the following capsule in due course if the following capsule is advancing at a higher speed. In the alternative, with the most advanced capsule advancing at a greater speed, the inter-capsule spacing will increase so that the two capsules increasingly separate one from the other. Capsules remain intact by acceptance of lead vehicle conduct by following vehicles, either voluntary or compelled by specific highway conditions.
The presence on a highway of the complex array of different vehicle travel characteristics mentioned above is most noticeable on principle routes where space (eg multiple lanes), high speed limits and multiple carriageways which eliminate contra flow conditions accommodate widely ranging exercise of choice in discretionary driving.
The availability of choice to vehicle drivers engenders a number of serious problems which in many cases are as apposite to increasing highway inefficiency as the increasing numbers of vehicles licensed to use the highways. UK motorways (and the broadly similar roads referred to by local nomenclature in other countries) and other principle traffic routes experience a number of sometimes remarkable problems engendered by exercise of choice by vehicle drivers. For example:                1. Spectacles, such as collision spectacles or even construction/repair spectacles, in one carriageway usually act as a virtual traffic flow constriction and give rise to a slowing of traffic in an adjacent carriageway to enable the drivers of the slowing vehicles to observe the spectacle. Slowing can be catastrophic causing multiple vehicle collisions in the carriageway experiencing slow-down. In any event, the deceleration of vehicles generates a deceleration wave as successive vehicles respond to reductions in inter-vehicle distances. Vehicles close in sequence to the lead vehicles may decelerate in a controlled fashion, possibly aided by an alert given by evidence in the adjacent carriageway of the spectacle itself. As driver alertness and vehicles distances vary from one driver/vehicle to another, the highway will inevitably experience the comparatively precipitous deceleration of one or more vehicles, and this produces a tail-back envelope of similarly precipitously decelerating vehicles many of which will decelerate to a speed substantially slower than the lead vehicles with some coming to a standstill. Slow speed conditions of the highway may render it incapable of absorbing extant traffic volume pressures, highway capsules in the tail emanating from the lead vehicles being forced to stasis as they cannot be admitted to more forward parts of the highway. Separate capsules tend to merge on slow-down, reforming with different characteristics and composition and usually again undergoing merger until the virtual constriction has been cleared.        2. The majority of drivers seek speed in the belief that this will result in efficiency of travel. However, data shows that safe vehicle distances at speed mean that a highway capsule progressing at speed s1 and containing n1 vehicles safely distanced at safe distance d1 advances more vehicles per unit time than a capsule progressing at a higher speed s2 and containing a smaller number of vehicles n2 safely distanced at larger safe distance d2.        3. Many divers engage in multiple lane changing upon a perception that different lanes in congested conditions advance at different speeds. However, tests show that multiple lane changing achieves little for the vehicle concerned, accelerates driver fatigue and can slow other vehicles.        4. A lead vehicle in a highway capsule dictates the speed of the capsule. Discretionary driving can thus lead to damage to traffic flow efficiency when a vehicle maintains such a commanding position whilst at the same time advancing at a speed less than the highway conditions will permit. Such a vehicle usually characterises the capsule it leads as one which has a void of unoccupied highway beyond its head.        5. Multiple lane highways usually are configured with the intention or acceptance that different lanes will be used by vehicles of different speed. Thus, for example, a UK motorway has in general three lanes with the outermost lane (on the right) used by relatively fast vehicles in overtaking mode. In relatively congested conditions, such vehicles tend to occupy that lane permanently or semi-permanently in increasing numbers, encouraged by the conviction that this will result in higher average speed for the vehicles concerned, at the expense of traffic volumes in the remaining lanes, particularly the inside lane (on the left). In very many cases, transfer of vehicles to one of those two lanes will enable an increase in the discharge of traffic by the highway as a whole because the two inner lanes are otherwise operating at inefficiently low traffic density with highway traffic capsules having low vehicle counts. This is particularly so where inner lanes are characterised by highway traffic capsules having voids of unoccupied highway beyond their heads.        6. Under jam conditions, the vehicles making up the jam and in common lane form a compacted supercapsule which is in stasis or in crawl with vehicles not in top gear. Removal of the cause of the jam releases the supercapsule which begins to decompact starting at its leading edge. Alert drivers tend rapidly to accelerate during decompaction and are commonly motivated, by a desire to compensate for the delays of the jam, to do so prematurely and excessively. Other drivers do not do so but participate with their vehicles in decompaction in a retired manner which may obstruct vehicles to their rear. Differences in driver attitudes in decompaction cause the fragmentation of the supercapsule to form plural separate traffic capsules in the manner referred to earlier. Under conditions of premature and/or excessive acceleration during supercapsule decompaction, inter-vehicle spacing is tolerated which ordinarily would not be accepted by drivers in exercise of discretionary driving. Indeed, driving is generally effected with a higher than usual degree of recklessness. This at worst predisposes the highway to collisions between vehicles which detract from highway safety and which also engender further jam-producing highway obstruction; at best, this recklessness leads to driver tension which predisposes drivers to precipitous deceleration of one or more vehicles causing production of a tail back envelope of similarly precipitously decelerating vehicles.        